Polarization can be used to encode images for stereoscopic three-dimensional (3D) displays. Left and right eye images are encoded with orthogonal polarization states, which match the transmissive states of left and right polarized lens eyewear worn by viewers. Since the polarization states are orthogonal, the leakage of the left/right images into the wrong eye can be minimized.
A disadvantage of the polarization method is that a minimum of 50% of light may be lost when a polarizer is used in the path of unpolarized light. A polarized light source, or an efficient conversion of the unpolarized light into polarized light, can be used to eliminate this loss. This can be effective for displays with modulators designed to process polarized light. For displays using modulators that were not designed to process polarized light, however, this may not work because the polarization state is not maintained through the system. Light in display systems using modulators that are not designed to process polarized light is polarized after the light has been modulated; however, less than 50% of the image light is used.
One successful display system that can be used with unpolarized light includes a digital micromirror device (DMD) provided by Texas Instruments Inc. of Dallas, Tex. There are a number of reasons why it has not been possible to improve the efficiency of a 3D stereoscopic display using DMDs by polarizing light before the DMD. Stress birefringence in the DMD window may alter the polarization state of the light. Furthermore, the stress varies across the window. Thus, the polarization can change non-uniformly across the active area of the DMD modulator. When the light polarization state has changed to become spatially non-uniform, light output may be reduced and light distribution across the display may be changed, both of which are undesirable. Additional complications arise for systems that employ multiple DMDs since the birefringence may not be the same from one DMD window to another. Additionally, the color prism used to split the light to the DMDs may exhibit a wavelength dependent polarization change.
One approach to recover the unused portion of image light that has been polarized after the DMD splits imaged light into two orthogonally polarized paths of imaged light. One path of polarized imaged light is directed to the screen and the other path of polarized imaged light is passed through a retarder and then reflected towards the screen to be superimposed on the imaged light from the first path on the screen. The retarder changes the polarization state of the imaged light in the second path to match the polarization state of the light in the first path so that all of the imaged light from the DMD is utilized. For this technique to be successful, imaged light from the second path is aligned with the imaged light from the first path on the screen. Any optical magnification, optical offset, or optical keystone effect that is different between the first and second imaged light path can result in a misalignment in the superimposed images on the screen and in a less than optimum presentation.
Accordingly, there remains a need for systems and methods that can allow a DMD-based display to operate with polarized light and to “repair” changes to the state of polarization.